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All summer, a single black-mohawked Steller's jay plied the root from a large pine by the pond across the street to my peanut feeder. He never had company, and, like the little gray squirrel in my yard, he was master of his territory. Now that fall has arrived, he has two Steller's jay friends. Each day, one summons the others when I finally get around to putting the feeders out. The squirrel remains alone and is busier than ever now that he has to chase a flock of jays instead of just one. A small flock, to be sure, but one that demonstrates a phenomenon that, in its larger forms, includes geese flying south in military V formations, and masses of swallows and starlings clouding the fall and winter skies in dramatic displays of aerial virtuosity.

In the spring, my jay was alone, guarding his territory. This solitary life is typical of many bird species during the time of year devoted to reproduction and launching baby birds into the world. Once these tasks are accomplished, territoriality fades. Birds become more gregarious and forage in groups. For the birdwatcher, flocking is a mixed blessing since an hour with the binoculars is sometimes wasted searching old familiar spots where summer singletons hung out. Stumbling on a foraging flock, however, can be very rewarding. Some flocks are mixed and include variations on the main bird theme as well as oddballs standing out because of differences in size and shape, as well as location on the crowd's periphery. Search a flock of ordinary herring gulls and you might find a guillemot.

In the natural world, we assume that most behaviors are related, one way or another, to survival and reproduction. Since flocking for some species appears only after reproductive tasks are completed, we can assume that togetherness serves their survival. Survival involves eating, not being eaten, and finding protection against the elements. As food supplies dwindle, many eyes searching are better than a few. Predators zeroing in on one bird can be thwarted when the entire flock takes off or shifts direction. And birds huddling together at night in old nesting cavities are better able to fend off the cold.

These flocking functions are easy to understand and make the topic seem pretty pedestrian. But some ornithologists suspect that more complex factors are also on display, related to social hierarchy and choice roosting spots. They study large flocks - huge numbers of birds, not just geese forming elegant V-shaped formations high above, but starlings and swallows massing in undulating clouds to travel from foraging sites back to roosting areas. The birds swoop back and forth across the sky at 40 mph, spiraling up and down and sideways in leaderless masses so dense that a Tour de France-style peloton crash seems inevitable. But it never happens. The show continues over the roosting site until some mysterious signal triggers the end. Then the birds spill out of the sky to settle in for the night, with the dominant birds winding up in the best spots in the trees or meadows. The last ones in are also the first ones out for work the next day.

So complex is flocking at this level that in the 1980s it inspired computer models of coordinated motion using three-dimensional computational geometry. The generic simulated flocking creatures are called boids, and movie director Tim Burton first used them in a film in 1992, turning them into flocks of bats. As Good as computer simulations are, they do not account for the speed of some flock maneuvers, and it took better photography, combined with computers, to show that the flock gyrations are similar to a human wave running through a stadium. They depend on each bird responding to the movements of six or seven other birds nearby - no more, no less. My Steller's jays will have to acquire more friends before they try any fancy moves.